Hybrid drives gain in importance in the vehicle manufacturing because of their potential to cut emissions and energy consumption. Such cars have various power sources, with combinations of internal-combustion engine and electric motors being particularly advantageous, because they can utilize, on the one hand, the range and performance advantages of internal-combustion engines and, on the other, the flexible applications of electrical machines as a sole or auxiliary power source or as a starter generator to produce electricity and ensure recuperation.
The market demands hybrid drive trains, which can be implemented into vehicles without additional space requirements, while minimizing the complexity and at low cost and design effort. In principle, we distinguish two hybrid topologies: the series hybrid and the parallel hybrid versions. Such arrangements are already known and are constantly further developed.
In the series hybrid, drive units are connected in series. Here, it is the internal-combustion engine, such as a diesel engine, that drives a generator, which then feeds an electric machine. The vehicle is thus driven solely by the electric machine. The internal-combustion engine, however, is decoupled from the drive wheels and can therefore be run always in a single operating point, i.e., at a given torque and constant speed. This drive concept is suitable, for example, for short journeys in urban buses, with the operating point preferably set to such level, at which the efficiency of the combustion engine is as high as possible while emissions, fuel consumption and noise lie in a favorable range. An unfavorable aspect of the series hybrid is, however, that the efficiency of the drive is limited because of the multiple electrical-mechanical energy conversion.
In contrast, by their parallel arrangement of the power flow through the power train unit and the superposition of the driving moments, hybrid power trains offer the possibility of driving the vehicle by purely internal-combustion engine or by purely electric-motor drive. Functionally, in a parallel hybrid, the combustion engine can be operated largely with an optimum torque due to the burden and/or support by one or more electrical machines so that the maximum efficiency point of the combustion engine can effectively be used. The electrical assistance to the combustion engine reduces on average fuel consumption. Since during short-term increased power demands in the so called boost mode, for example, when overtaking, a summation of the drive power is possible, the internal-combustion engine can be designed relatively small, saving weight and installation space with almost no loss of performance and driving comfort of the vehicle, which in turn reduces emissions and lowers cost. In addition, the electric machine can also act as an integrated starter generator (ISG) to start the internal-combustion engine via a coupling. Furthermore, the electrical machine in generator mode can be used to charge an electrical energy storage device and to recuperate energy when the vehicle is braking. In principle, all types of vehicle transmissions can be considered for the purpose of variation of translations of the drive.
In a parallel hybrid drive, depending on the particular hybrid operation strategy, one can switch during the actual driving between the internal-combustion engine drive, electromotor drive or mixed drive. The changing connection of the electric motor and the combustion engine is usually implemented using clutches. We distinguish two-clutch arrangements (2C) and one-clutch arrangements (1C), where with both concepts, the electric motor can act as an integrated starter generator (2C-ISG or 1C-ISG arrangement). In a 2C-ISG power train, such as the one known from US 2005 022 1947 A 1, the internal-combustion engine can be connected, via a first clutch, to the electric motor. In turn, the electric motor is coupled via a separate second clutch to a vehicle transmission. In a 1C-ISG power train, as it is known for example from the DE 10 2005 051 382 A1, however, a second separate clutch between the electric motor and the gearbox or the output is eliminated. The electric machine can then be directly connected to the transmission. The function of an optional second clutch between the electric motor and the output may, where in the respective drive system provided or required, can also by assumed by any gear-box internal clutches and/or switching brakes, as they are built in, for example, automatic transmissions, or by a torque converter clutch arranged upstream the gearbox.
It is also known to implement a creep mode in vehicles with automated transmissions or automatic transmissions to increase the ride comfort and the operation reliability in the control and regulation of the vehicle. In this arrangement, a creeping moment is transmitted from the drive to output or to the driven vehicle wheels, while this creeping moment can be adjustable to a predetermined characteristic value or characteristic. In such an operating mode, the vehicle is moving, with a set gear ratio, a non-actuated brake and non-actuated accelerator pedal, with very low speed. On slopes, one can perform a temporary and limited stop operation using the creep mode.
Depending on the drive concept, a creep mode can be implemented and controlled in the drive train in different ways. In conventional vehicles with an automated gearbox and an automatic frictionally engaged clutch, the creep mode can be realized by appropriate control of the starting clutch. With other conventional vehicles with automatic transmissions and a hydrodynamic torque converter, a creep moment produced by the torque converter is primarily determined by the idling speed of the internal-combustion engine. For hybrid or electric vehicles, as an alternative to conventional friction clutches or hydrodynamic torque converters, an existing electric propulsion device can also be sued to produce a creep mode.
Compared to a power train with a frictional clutch, which is operated in creep mode in order to transmit the creeping, such electrical creeping operation has, in principle, the advantage of lower mechanical losses incurred in power transmission. It also eliminates the risk of overheating the clutch. It therefore seems to be obvious in a hybrid vehicle to use a purely electric creep generated by the electric machine when the internal-combustion engine is running at idle and is uncoupled from the power train.
It is however problematic that the electrical drive energy storage device of the hybrid vehicle can empty itself with the electric motor operating as the driving motor in a relatively short period of time so far that the creeping operational mode must be stopped or interrupted in order to load the energy storage device in the generator mode of the electric machine. Thus, the electric machine would temporarily not be available to drive the vehicle and, where appropriate, to supply additional appliances. Therefore, a permanent electric motor creeping in a hybrid vehicle is to be assessed as a non-optimal solution. On the other hand, in the case of a permanent creeping through a slip clutch there may arise high power losses in it that require an appropriate construction size with increased space requirements and additional weight as well as a relatively expensive cooling system.
DE 101 58 536 B4 discloses a drive for an electric or hybrid vehicle, in which an operational creeping is realized by means of an electric drive unit. In the power train of the vehicle, a clutch device is arranged between the electric motor and an output of a clutch device. The clutching device may, for example, be designed as a direct-drive clutch, which technically precedes an automatic transmission. Alternatively, one or more clutches and/or switching brakes of an automatic transmission can function as the said coupling device. To reduce the thermal load on the electric motor in the creep mode at high torque requirements, such as when the vehicle is crawling or stopping at slopes, or when driving over curbs, and at the same time to avoid major dimensions of the electrical machine and/or the coupling device, there can be operated a clutch of the clutch device in the creep mode parallel to the motor operation of the electric motor as necessary. If the output side of the clutch device comprises multiple clutches, they may also be used alternately or in addition the creep mode. If the electric machine comprises two separate excitable windings, then these can be operated in constant change. A combination of these means or measures can avoid a thermal overload of both the coupling device (clutch) and the electric machine at high driving torques at very low speeds or with a high permanent static torque. A disadvantage of this is that during the permanent creeping the electric machine is continuously driven by the motor for a relatively long period. It is true that the electric machine can be supported by the transmission-side coupling device and /or by the several excitable windings that are controllable in a changing mode. The relatively intensive use of the electric energy storage drive of the electric machine can, however, still significantly restrict such a permanent creeping operation. In addition, an electrical machine with several separate windings may also be relatively expensive to manufacture. In addition, the complete abandonment of the operation of the combustion engine during the permanent creeping with a hybrid drive is rather ineffective.
Against this background, the technical task of the invention is to provide a method and an apparatus for controlling a creeping operation of a vehicle with a hybrid drive that allows to create a simple, effective and design-saving permanent creeping without any additional construction cost.